1、 Technical principles

COD pre made reagent is a method of adding a known amount of potassium dichromate solution to a sample, in a strong sulfuric acid medium, using silver sulfate as a catalyst, and after high-temperature digestion, the COD value is determined by spectrophotometry. Measure the total absorbance of unreduced hexavalent chromium and reduced trivalent chromium produced by potassium dichromate at a wavelength of 420nm; The COD value in the sample is directly proportional to the decrease in absorbance of hexavalent chromium, the increase in absorbance of trivalent chromium, and the decrease in total absorbance. Therefore, the total absorbance value can be converted into the COD value in the sample.

2、 Scope of testing

5-150 mg/L (for undiluted water samples).

3、 Application Fields

Used for the determination of COD values in water samples such as surface water, groundwater, domestic sewage, and industrial wastewater.

4、 Usage method

1. First, put on a pair of clean gloves, take out at least 2 pre installed tubes and unscrew the covers. Add 2.00 mL of ultrapure water to one tube as a blank, and add 2.00 mL of the test sample solution to the others.

2. After adding the sample solution, tighten the lid firmly (cover each tube back to its original lid), hold the tube lid upside down and shake it several times to mix the sample solution and pre loaded reagent evenly. During the mixing process, a lot of heat will be released. Be careful to wear gloves to prevent overheating.

3. Place the blank tube and pre loaded tube with added water sample into a porous digester that has been preheated to 165 ℃. The heater temperature should be slightly lowered, and when the temperature rises to 165 ℃, the digestion should be timed for 20 minutes.

4. Take out the digestion tube from the digester and wait for it to cool down to about 60-80 ℃. Hold the tube cover upside down and shake the digestion tube several times to make the solution inside the tube uniform. Use a lens cleaning paper or a clean cloth to clean the outer wall of the tube, then let it stand and cool to room temperature. After the mercury sulfate completely settles to the bottom, use a blank tube to zero the absorbance of the test sample at a wavelength of around 420 nm, and calculate the COD value based on the existing standard curve.

5、 Method for drawing standard curves of COD prefabricated reagents

① Preparation of 1000 mg/L COD standard solution: Dry the reference grade potassium hydrogen phthalate (KHP) at 105~110 ℃ for more than 3 hours, cool it to room temperature in a dryer, accurately weigh 0.8509 g of dried KHP with an analytical balance, dissolve it in ultrapure water and make up to the 1000 mL mark on a volumetric flask to obtain a 1000 mg/L COD standard solution;

② Preparation of standard series solutions: Transfer 2.50, 5.00， 10.00， Transfer 15.00 mL of 1000 mg/L COD standard solution into four clean 100 mL volumetric flasks, dilute with ultrapure water to the mark and shake well to obtain 25, 50, 100, and 150 mg/L standard series COD standard solutions, respectively;

③ Draw the standard curve: take five COD pre installed tubes, add 2.00mL ultra pure water, 25, 50100 and 150 mg/L COD standard solution respectively, cover them, shake them evenly upside down, operate according to the use method in step 5 above, and the absorbance A corresponding to 0, 25, 50100 and 150 mg/L COD standard solution can be obtained. Use the concentration C of COD standard solution as the ordinate, and the corresponding absorbance A as the abscissa for drawing. The formula of the standard curve can be obtained by linear fitting, and the measured absorbance of the actual sample can be substituted into the formula to calculate the COD value (if the instrument supports a self built test program, the standard curve can be input into the program, and the The program test can directly display the COD value without further calculation).

6、 Precautions

1. If the photometer used has a circular hole sample cell that matches the pre installed tube, the pre installed tube can be directly used as a colorimetric tube; If not available, the digested liquid can be aspirated into a 1cm colorimetric dish for further testing;

2. If the user's laboratory has Hash DR series instruments (such as DR3900/DR6000/DR900/DR5000/DR2400, etc.), there is no need to draw a standard curve, and the COD 20 min LR digestion test program can be directly called for zeroing test (program number 436);

3. This pre installed tube is suitable for water samples with a chloride ion concentration not exceeding 1000 mg/L (diluted), otherwise it should be diluted according to the mass ratio of m [HgSO4]: m [Cl -]

Add an appropriate amount of solid mercury sulfate at a ratio of ≥ 20:1;

4. Blank and test samples should be made using small tubes of the same batch number for each test, and the batch number can be found on the product label;